— In a move that was part Byzantine intrigue, part schoolyard squabble, the U.S. Senate effectively scuttled two bipartisan energy proposals Monday, including one to build the Keystone XL pipeline that was co-sponsored by U.S. Sen. Mary Landrieu, D-La.

“Common sense — that is in short supply in the United States Senate, and it’s very disappointing,” Landrieu said on the Senate floor after the vote.

The actual decision by the Senate came in a failed vote to cut off debate on an energy-efficiency bill. The measure, whose principal sponsors are U.S. Sens. Jeanne Shaheen, D-N.H., and Rob Portman, R-Ohio, would tighten energy-saving guidelines for new federal buildings and offer tax incentives for making homes and commercial buildings more energy-efficient.

A successful vote to end debate would have opened to way to Senate approval of the bill with a simple majority of 51 members. The 55-36 vote to close debate fell five votes short of the 60 needed for that action.

Senate Majority Leader Harry Reid, D-Nev., has stymied amendments to the bill, including one backed mainly by Republicans that would compel federal approval of the proposed Keystone XL pipeline from Canadian oilfields to refineries on the Texas Gulf Coast.

The project has been a flashpoint between environmentalists and oil-industry allies. Keystone supporters — including both Landrieu and the main Republican challenger to her re-election this fall, U.S. Rep. Bill Cassidy, R-Baton Rouge — say Keystone would create jobs and strengthen the nation’s energy infrastructure.

Opponents say it represents a step backward in terms of reducing carbon emissions, especially as it would transport relatively “dirty” oil from tar sands in Alberta, Canada.

The power to approve or reject the pipeline currently rests with the White House, which has put off a decision for years. A further delay announced by the Obama Administration in April because of a legal squabble in the Nebraska state courts over the pipeline’s route.

Republicans also want to amend the Sheehan-Portman bill to block the Environmental Protection Agency from imposing stricter greenhouse-gas standards on coal-burning power plants.

Reid proposed an up-or-down vote without additional amendments on the energy bill first, promising a vote afterwards on a separate build-Keystone bill co-sponsored by Landrieu and Sen. John Hoeven, R-N.D. Senate Republican leader Mitch McConnell of Kentucky — himself facing a re-election challenge — has objected to that plan.

“The Republican leader wants an issue,” Landrieu said on the floor. “He does not want the pipeline. I hope the people of Kentucky will remind him how important the pipeline is.”

She described as “bogus” attempts by Republicans to blame failure of both bills on the Democrats refusal to consider Sheehan-Portman amendments.

But in a statement issued after the vote, Cassidy pressed that partisan view.

“Approving the Keystone pipeline would bring thousands of jobs to Louisiana,” he said. “It is disappointing that Senate Democrats are unwilling to compromise to make this happen.”

A separate Keystone bill could be filibustered by its opponents, meaning that, again, 60 votes would be required to end debate and advance it. Even if passed, it could be vetoed by President Obama, and it would take 67 votes in the Senate to override the veto. Despite support for Keystone from a number of his fellow Democrats, including Landrieu, such a veto likely would be an easier one for the president to cast than a veto of an energy-efficiency bill with a build-Keystone amendment attached to it — especially as the president favors many of the provisions of the Shaheen-Portman measure.

Republicans earlier agreed to Reid’s now-and-later plan, with the prospect of Obama taking at least some heat for vetoing a popular, stand-alone build-Keystone measure. But they reversed course, which has clearly rankled Landrieu. A three-term senator recently named chairwoman of the U.S. Senate Energy and Natural Resources Committee, she has been fending off claims by Cassidy that her failure to win approval for Keystone demonstrates her ineffectiveness in Washington.

In announcing the day’s schedule that would include the vote on the Shaheen-Portman bill, Reid praised the measure as “a good first step” toward solving the nation’s energy problems and as a tool to create 200,000 jobs -- but he attacked the legislative strategy of the Senate Republicans that has left the bill in limbo.

“They’ve held this bill hostage,” Reid said. “Demand after demand has been met but even now, they are still seeking ransom.”

A request for an explanation of Reid’s refusal to allow a build-Keystone amendment did not yield a response from his office.

Landrieu voted for the debate cut-off; her Republican seatmate, David Vitter, missed the vote.

Vitter was in Baton Rouge for meetings with state legislators.

Reid actually voted against the cutoff proposal, in a procedural move that allows him to bring the proposal up again.

Landrieu’s office issued a statement after the vote saying she will continue to press for approval of the pipeline.